1. Field of the Invention
The present invention relates to a liquid crystal electro-optical device and a method for manufacturing the same.
2. Description of the Prior Art
Ferroelectric liquid crystal display devices which are now internationally being researched are superior in high response speed, wide angle of visibility, and the like in comparison to liquid crystal display devices of TN (Twisted Nematic) type which have been used for watches, electric calculators, and the like for years.
The high response speed of ferroelectric liquid crystal results from spontaneous polarization which liquid crystal itself has. Generally a response time of ferroelectric liquid crystal is in inverse proportion to spontaneous polarization and external electric fields and is in proportion to viscosity. So that, conventionally ferroelectric liquid crystal whose spontaneous polarization was small was useless due to slow response speed, and the development of ferroelectric liquid crystal which has large spontaneous polarization is in progress. However, recently it was found that this spontaneous polarization has a large influence on switching of ferroelectric liquid crystal.
Namely, because of the presence of the spontaneous polarization, polarization charges produce an electric field in a ferroelectric liquid crystal layer even in the condition where an electric field is not applied from outside and ionized impurities such as sodium ions, potassium ions, and lithium ions are unevenly distributed in the ferroelectric liquid crystal layer. Consequently the ionized impurities restrain the direction of the spontaneous polarization of ferroelectric liquid crystal molecules, so that the ferroelectric liquid crystal molecules which show bistability in an ordinary condition cannot help being monostable, which causes the degrade of ratio of contrast.
As orientation film for orienting liquid crystal molecules in a liquid crystal device is used a silicon oxide film obliquely formed by evaporation, organic macromolecule compound film such as polyimide film and polyamide film, and the like. Particularly, a polyimide film is widely employed since it is superior in heat resistance and it is easily formed. A solution of polyimide is applied on a substrate and a solvent in the solution is removed, and then the substrate is subjected to a thermal treatment if necessary, to thereby form a polyimide film. Subsequently this polyimide film is subjected to rubbing treatment, and thereby an orientation film is obtained.
However in a ferroelectric liquid crystal electro-optical device using strongly insulating films such as polyimide film, silicon oxide film, or the like as orientation films, the ionized impurities in the liquid crystal are unevenly distributed on interfaces between the liquid crystal layer and the orientation films when dipoles of the ferroelectric liquid crystal molecules are aligned, and consequently uneven distribution of electric charges is produced in the device.
As the result, for example, when a fixed display is maintained for a few hours in a ferroelectric liquid crystal display device and subsequently is changed to a different display, the fixed display still remains (the remaining display is referred to as after-image hereinafter). This is a big obstacle for display devices.
In order to solve this problem, resistance of the orientation films was made low. For example, in Japanese Patent Provisional Publication No. sho62-295028 is disclosed liquid crystal display devices which are provided with orientation films made from polyimide mixed with conductive particles such as metallic powders or conductive organic compounds.
Generally, orientation films provided in ferroelectric liquid crystal display devices are superior in switching when the thickness of the orientation films are 1000 .ANG. or less, preferably 500 to 200 .ANG.. However, it is extremely difficult to disperse the conductive particles uniformly in such a thin film.
Further, some of the conductive particles mixed in orientation films are naturally bared on the surfaces of the orientation films. As time passes, such conductive particles enter the liquid crystal layer and thereby ionized impurities are increased in total in the liquid crystal layer, and as the result the after-image is increased. In addition, by the increase of the ionized impurities, an electric current flowing in the liquid crystal layer is increased, which causes deterioration of the liquid crystal material and degrades the reliability of the device. Furthermore, due to this increase of the electric current, the power consumption is increased.
Generally, liquid crystal contains a few ionized impurities since reagent used in a process of synthesizing the liquid crystal and secondary products are not completely removed. So that, a method of purifying repeatedly the liquid crystal by means of recrystallization or a method of purifying the liquid crystal by means of zone melting method would be adopted as a method for decreasing the ionized impurities in the liquid crystal. However, these purifying methods need much time and have low yield. Therefore, cost is raised.
Even though the liquid crystal is sufficiently purified, impurities enter the liquid crystal material while manufacturing the liquid crystal device. Further, ionized impurities enter there from the orientation films and sealed portion even after the device is completed. Therefore, it can not be avoided that ionized impurities enter a liquid crystal layer.
Such ionized impurities existing in a liquid crystal layer provided in a liquid crystal electro-optical device such as a ferroelectric liquid crystal electro-optical device, a liquid crystal electro-optical device of TN type, and a liquid crystal electro-optical device of STN type hinder directions of liquid crystal molecules in the liquid crystal layer from being turned, which consequently degrades quality of display.